1. Field of the Invention
This invention relates generally to an information recording apparatus and, more particularly, is directed to an apparatus suitable for recording digital data on a magnetic recording medium.
2. Description of the Prior Art
Apparatus for recording digital data on a magnetic recording medium is already known, for example, in the form of an 8-mm format video tape recorder (VTR) which is used for recording a PCM audio signal. In such 8-mm format VTR, two rotary magnetic heads are used to record respective slant tracks TRA and TRB, respectively, which are disposed alternately on a magnetic tape 1, as shown on FIG. 11. Further, in the 8-mm format VTR, the audio signals are PCM-encoded, time-based compressed and recorded in a respective portion TR.sub.AD of each of the slant tracks TRA and TRB, while the remaining portion TR.sub.VD of each of the slant tracks may have a field of the video signal recorded therein. A particular type of modulation used to record the time-compressed, PCM-encoded audio signal is biphase-mark modulation, for example, as described in detail in U.S. Pat. No. 4,551,771, issued Nov. 5, 1985, and having a common assignee herewith.
As shown in FIG. 12A, in biphase-mark modulation, frequency signals S.sub.1 and S.sub.2 having frequencies f.sub.1, for example, of 2.9 MHz, and f.sub.2, for example, of 5.8 MHz, are recorded as digital data on the magnetic tape while maintaining a phase relationship in which their zero cross points coincide with each other. For example, when a PCM audio signal is at logic "L", the frequency signal S.sub.1 having the low frequency f.sub.1 is recorded, whereas, when the PCM audio signal is at logic "H", the frequency signal S.sub.2 having the high frequency f.sub.2 is recorded on the magnetic tape.
When an audio signal recorded in an audio track portion TR.sub.AD is reproduced or played back the zero cross points of the picked-up frequency signal S.sub.1 or S.sub.2 are detected, as shown in FIG. 12B. In response to such detection, for example, a sampling pulse oscillator with a phase locked loop (PLL) arrangement is made to oscillate and thereby provide output pulses PLL.sub.O which are phase shifted 45 degrees from the frequency signal S.sub.1 having the low frequency f.sub.1, or phase shifted 90 degrees from the frequency signal S.sub.2 having the high frequency f.sub.2. Those of the output pulses PLL.sub.O generated at the phase angles of 45 degrees, 135 degrees, 225 degrees, and 315 degrees of the frequency signal S.sub.1 are used as sampling pulses P.sub.S (FIG. 12C) for sampling the reproduced data. When every other two of the resulting sampled values are positive, it is determined that the respective playback data is at logic level "L". Further, those of the output pulses PLL.sub.O generated at the 90 degree and 270 degree phase angles of the frequency signal S.sub.2 are also used as sampling pulses P.sub.S (FIG. 12C) for sampling the reproduced data and, when every other of these sampled values are positive, it is determined that the respective reproduced data is at logic level "H". The reproducing of biphase-mark modulated data is further described in U.S. patent application Ser. No 115,045, and in the corresponding EPO laid-open publication No. 0264228, which also have a common assignee herewith.
As shown in FIG. 13, the video signal S.sub.VD recorded in the video track portion TR.sub.VD of each of the tracks TRA and TRB is known to consist of an FM luminance signal S1V, an FM audio signal S2V lower than the signal S1V and a low frequency or down-converted signal S3V. An automatic track follower (ATF) signal S.sub.ATF is superposed on the video signal S.sub.VD for recording therewith in the video track portion TR.sub.VD. The same rotary magnetic head used for recording the video signal S.sub.VD is also used for recording the PCM audio signal S.sub.ADNR in the audio signal portion TR.sub.AD of the respective track. The frequency spectrum of the PCM audio signal S.sub.ADNR is shown on FIG. 13 to have a peak value at a frequency which substantially coincides with the center frequency of the FM luminance signal S1V. Further, the PCM audio signal S.sub.ADNR has a frequency characteristic in which the signal level nears zero in the high and low frequency ranges of the video signal S.sub.VD. By reason of the foregoing, the same degrees of azimuth loss are obtained when one of the rotary magnetic heads reproduces the video signal S.sub.VD and the PCM audio signal S.sub.ADNR recorded in respective portions of the same slant track. Therefore, the PCM audio signal S.sub.ADNR can be recorded with no guard band between adjacent tracks, and with the two rotary magnetic heads having gaps with different azimuth angles so that the azimuth loss can be relied upon for substantially minimizing cross talk between the PCM audio signals recorded in adjacent tracks.
Depending upon the application thereof, digital data recorded on a magnetic recording medium may have different sampling frequencies. Therefore, recording media of different types are used for recording digital data having respectively different recording densities. If a reproducing or playback apparatus can be adapted for reproducing data from the different types of tapes having respectively different recording densities, the uses of such playback apparatus will be accordingly extended. Thus, for example, it is advantageous to provide an 8-mm VTR capable of use with first and second types of recording media, in which the first type of recording medium records PCM audio data having a standard digital information content sampled at a standard sampling frequency, while the second type of recording medium records PCM audio data having a relatively larger content of high-quality digital data sampled at a frequency that is a predetermined multiple of the standard sampling frequency. If these first and second types of recording media could be reproduced in a common 8-mm VTR, that is, the same apparatus can be used for reproducing a standard-quality audio signal from the first type of recording medium and for reproducing a high-quality audio signal from the second type of recording medium, the 8-mm VTR will be of substantially expanded utility However, if the same recording current is supplied to a magnetic head when recording data on different types of magnetic recording media having different recording densities, the bit error rate may be undesirably increased when the recorded data is reproduced